A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient N and O Atoms

solvents in the absence of added catalysts. Also, the stronger the acid
XOH, i.e. the more stable the anion eOX, the more readily ®OX
should be lost to yield (XXIX) and the more rapid the reaction should
be. This is borne out by the fact that the rate of reaction increases in
the series where XO® is CH 3 C0 2 ® < CICH 2 C0 2 ®< PhSOs®.
That such ionisation is the rate-determining step in the reaction is
also suggested by the observed increase in the rate of reaction as the
solvent is made more polar.
It is not certain, in either case, whether the fission of the N—O
bond and migration of R are actually simultaneous but, if not, the
rearrangement follows extremely rapidly after the fission for it has
been demonstrated that the migrating group attacks the back of the
nitrogen atom, i.e. the side remote from the leaving group, and that
if R is asymmetric it migrates without undergoing any change of
configuration. In addition, no cross-migration of R groups has been
observed when two different, but similar, oximes are rearranged
simultaneously in the same solution, i.e. this is another intramolecular
rearrangement in which R never becomes wholly detached from the
molecule.
After migration of R, the rearrang£ment is completed by attack of
water on the positi^ carbon (it is, of course, at this stage that lsO is
introduced in the rearrangement of benzophenone oxime referred to
above), followed by loss of proton to yield the enol of the amide
which then reverts to the amide proper.
The stereochemical use of the Beckmann rearrangement in assigning
configuration to ketoximes has already been referred to and one
large-scale application is in the synthesis of the textile polymer,
perlon:

OH H OH O

II

o

NOH

NH.-OH

H®

o H

Perlon